The audio level is captured by the Plivo SDK. Call legs that have a PSTN number will not display the audio level. One-way audio cannot be detected on a PSTN leg if the audio level is too low on one of the streams. However, the audio level can be detected in case if the packet count on one of the streams is zero.
If both the streams have a packet count of zero, the call leg will be labeled as “no audio”.